With the continuous aggravation of the resource and energy crisis, topics such as being environmentally friendly, sustainable development, green economy, low carbon economy gets increasing attention, and the national “12th five-year” plan also clearly states that energy conservation and emission reduction will be taken as an important task, which raises a severe test for the iron and steel industry with respect to the traditional industries.
In the mid-nineteenth century, British Henry Bessemer first proposed a method for casting a metal thin strip using a twin-roll continuous caster, i.e., a technique for directly casting molten steel as a thin strip billet so that the intermediate billet casting and hot rolling procedures are saved. With the continuous improvement of techniques, the envisage of Bessemer is widely developed in the recent 30 years, iron and steel enterprises all over the world establish thin strip continuous casting experimental units one after another, and among them, some also realized industrial productions; and such a technique for directly casting molten steel as a thin strip billet is mainly used for the manufacture of stainless steels, high-speed steels and nonferrous metals. Among various thin strip continuous casting techniques, a horizontal isodiametric twin-roll thin strip continuous casting technique is the most maturely developed technique, and the method can produce a thin strip billet having a final thickness of 2-6 mm.
As compared to traditional sheet production process routes, the thin strip continuous casting technique has characteristics, such as a short flow, a small investment of capital construction, achieving “once heating production”, etc., which substantially improves the energy efficiency of steel production and saves the production cost. However, the surface quality of a product from the thin strip continuous casting is not high, and subsequent processing is required in general.
The process for hot galvanization, including hot plating with pure zinc and other alloy, is an important method for enabling steel and iron materials to be oxidation resistant and prevention from atmospheric corrosion, and hot-galvanized products have characteristics, such as a remarkable protection effect, a good surface quality, low costs, etc., and are widely used in industries such as automobiles, ships, constructions. Hot-galvanized products on the current market are mostly produced by continuous hot-galvanization production lines, and such processes have a long production cycle, wherein a metal needs to experience a plurality of procedures such as billet casting, hot rolling, pickling, cold rolling, etc., from molten steel to continuous annealing and hot-galvanization; it is these complex processing procedures that cause a difficulty of matching thin strip continuous casting with the hot-galvanization process, wherein the key obstacle comes from the pickling procedure, and the pickling procedure can further results in a serious pollution and damage to the environment.
Chinese patent application no. 201310489332.6 discloses a method for producing a hot-rolled pickling-free sheet by combining thin strip continuous casting with reduction annealing, and the general process route is: cast-rolling molten steel into a strip billet, hot levelling, then cooling and coiling, wherein in order to obtain the pickling-free product, it is necessary to uncoil a cast-rolled sheet coil, perform reduction annealing, and then coiling the finished product or perform hot galvanization, and the resulting hot-galvanized product is still obtained from separated lines with a plurality of procedures and lacking continuity. American U.S. Pat. No. 6,588,491 B2 discloses a method for directly producing a pickling-free strip steel, wherein the critical step is casting molten steel as a thin strip, reduction annealing (reducing iron oxide skin into a porous iron), and mechanically brushing away the porous iron so as to obtain a pickling-free strip steel; with this as a core, and by adding procedures such as hot rolling, cold rolling, oil coating, hot galvanization, electro galvanization etc. before and after this step, the production of various products is achieved. The defect is that the metal iron produced by the reduction is mechanically brushed away, which only increases the production procedure, but also reduces the metal recovery, thus increasing the production cost; in addition, the reduction is reduction at a certain holding temperature and then cooling, as shown in FIG. 1, i.e., a one-section temperature holding reduction procedure is used, and the main difficulties faced are all as follows: the iron oxide skin reduction rate is low, the efficiency is low, the reduction is not complete, and the product performance is affected; and reference can be made to FIGS. 2-5, wherein the reduction of the iron oxide skin in FIG. 2 is not complete, the iron oxide close to the surface is reduced, while a great amount of oxides are remained at the underlayer; FIG. 3 is large cracks present on the surface after the reduction of the hot-rolled sheet; moreover, the reduction efficiency is low, and a great amount of oxides on the surface are not reduced in time; the reduction of the iron oxide skin in FIG. 4 is not complete, the periphery of oxide particles is reduced, while the centre is still oxides; and the reduced iron in FIG. 5 is uniformly distributed on the surface, but has a dense structure and on the contrary blocks the transfer of the reducing gas, affecting the reduction of the oxides in the internal layer, and finally leading to a non-thorough reduction.
Therefore, there is an urgent need to provide a new method capable of achieving directly producing a pickling-free hot-galvanized sheet strip product from molten steel by a rational design and optimization, so as to enable a shorter flow, a higher reduction efficiency and a higher metal recovery.